Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752601Ab3EPLiS (ORCPT ); Thu, 16 May 2013 07:38:18 -0400 Received: from mail-wi0-f169.google.com ([209.85.212.169]:52971 "EHLO mail-wi0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752393Ab3EPLiR (ORCPT ); Thu, 16 May 2013 07:38:17 -0400 Date: Thu, 16 May 2013 13:38:12 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: Steven Rostedt , Don Zickus , Ingo Molnar , LKML , "Paul E. McKenney" , Andrew Morton , Thomas Gleixner , "H. Peter Anvin" Subject: Re: [PATCH 1/2] nohz: Disable LOCKUP_DETECTOR when NO_HZ_FULL is enabled Message-ID: <20130516113811.GB2341@somewhere> References: <1368547372-21011-1-git-send-email-fweisbec@gmail.com> <1368547372-21011-2-git-send-email-fweisbec@gmail.com> <20130515083729.GC10510@laptop.programming.kicks-ass.net> <20130515150652.GP23604@redhat.com> <1368631622.6828.69.camel@gandalf.local.home> <20130515165915.GE13916@laptop.home> <1368637441.6828.70.camel@gandalf.local.home> <20130516081027.GD19669@dyad.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130516081027.GD19669@dyad.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1809 Lines: 43 On Thu, May 16, 2013 at 10:10:27AM +0200, Peter Zijlstra wrote: > On Wed, May 15, 2013 at 01:04:01PM -0400, Steven Rostedt wrote: > > On Wed, 2013-05-15 at 18:59 +0200, Peter Zijlstra wrote: > > > > > At which point we could run the watchdog without perf_event_task_tick(). > > > > At which point we can drop the disable LOCKUP_DETECTOR when NO_HZ_FULL > > is enabled ;-) > > > > Can we? The thing I'm worried about is RCU (of course!). ISTR we rely on RCU > working in NMI context. AFAIR for RCU to work, we need to come out of out magic > NO_HZ state since that would've put RCU into EQS. > > Frederic, PaulMck? But they are protected inside rcu_nmi_*() functions, that's the only thing we need. If this interrupt userspace then we resume back to it quickly after the NMI and re-enter EQS. No need to restart the tick for that. A remote CPU that wants a quiescent state from the dyntick CPU will notice soon enough the EQS. We can certainly drop the perf tick for NMI watchdog: 1) As long as there are no flexible events competing on the CPU, no rotation should be needed. 2) We don't want event throttling for the watchdog. There is even a hack to handle that: /* Callback function for perf event subsystem */ static void watchdog_overflow_callback(struct perf_event *event, struct perf_sample_data *data, struct pt_regs *regs) { /* Ensure the watchdog never gets throttled */ event->hw.interrupts = 0; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/